Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems vary the amount of fuel delivered for combustion in response to multiple system inputs that can include, throttle angle, engine speed, mass airflow rate and the exhaust gas composition produced by combustion of air and fuel.
Electronic fuel control systems operate primarily to maintain the ratio of air and fuel at or near stoichiometry. Electronic fuel control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. Two primary modes of operation are closed-loop fuel control and open-loop fuel control.
Closed-loop fuel control is utilized when engine output required and exhaust sensor conditions both allow operation to lower harmful emissions. Under closed-loop control, the amount of fuel delivered is primarily determined primarily by an air charge estimate, which is the mass of fresh air captured in a cylinder. This estimate is then modified by a value related to the concentration of oxygen in the exhaust gas. The concentration of oxygen in the exhaust gas is indicative of the fuel-air composition that has been ignited.
In closed-loop operation, the exhaust gas oxygen is sensed by an oxygen sensor. Such an oxygen sensor may be of various types: including a Exhaust Gas Oxygen (EGO) sensor, a Heated Exhaust Gas Oxygen (HEGO) sensor, or Universal Exhaust Gas Oxygen (UEGO) sensor. The electronic fuel control system adjusts the amount of fuel delivered in response to the output of the oxygen sensor. A sensor output indicating a rich air/fuel mixture (an air/fuel mixture with fuel quantity above stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an air/fuel mixture with fuel quantity below stoichiometry) will result in an increase in the amount of fuel being delivered.
The fuel control system maintains adjustment or correction information concerning the amount of fuel required during closed-loop control for different engine speeds (engine angular velocity) and air intake rates. This information varies from engine to engine within a given family or type due to variations in parts, variations in rates of aging of parts, and the conditions under which the vehicle is driven. Consequently, the fuel control system continuously "learns" the different requirements of the engine, and operation under both open-loop and closed-loop control is enhanced.
The information is updated steadily while closed-loop fuel control is employed, and is utilized as a correction term to alter the fuel value generated by the fuel control system. Thus the "learned" information is used to achieve greater accuracy in the amount of fuel delivered to the engine.
Because of fuel control system memory or construction limitations, only limited amount of adjustment information can be stored. Consequently, information corresponding to the precise conditions under which the engine is operating is generally unavailable. In such cases, the information needed is determined by interpolating from the information stored for similar conditions.
Nominally, the relationship between fuelling and mass air flow rate is linear. However, multiplying the generated fuel value by the correction term to determine the fuel delivery rate can lead to errors because an offset in the relationship is affine rather than linear.